1. Technical Field
The invention relates to processes for preparing 3-aminomethyl-3,5,5-trimethylcyclo-hexylamine (isophoronediamine, IPDA) having a high cis/trans isomer ratio.
2. Discussion of the Background
IPDA is used as a starting product for preparing isophorone diisocyanate (IPDI), an isocyanate component for polyurethane systems, as an amine component for polyamides and as a hardener for epoxy resins. IPDA is customarily prepared from 3-cyano-3,5,5-trimethylcyclohexanone (isophoronenitrile, IPN) by converting the carbonyl group to an amino group and the nitrile group to an aminomethyl group in the presence of ammonia, hydrogen and customary hydrogenation catalysts. Mixtures of cis-IPDA and trans-IPDA are obtained. The two isomers have differing reactivities, which is of significance for the intended technical application. According to DE-A 42 11 454, the use of an IPDA isomer mixture consisting of above 40% of the trans-isomer and below 60% of the cis-isomer as a reaction component in polyaddition resins, in particular epoxy resins, both lengthens the pot life and reduces the maximum curing temperature. Conversely, to achieve a very high reaction rate, preference is given to IPDA isomer mixtures which have a very high cis-isomer content (≧70%). Commercially obtainable IPDA therefore has a cis/trans isomer ratio of 75/25.
Various processes for achieving a high cis/trans or a high trans/cis ratio are already known from the prior art.
According to DE-A 43 43 890, the animating hydrogenation of IPN to IPDA is effected by allowing a mixture of IPN, ammonia and a C1-C3-alcohol to trickle through a trickle bed reactor equipped with a cobalt and/or ruthenium fixed bed catalyst in the presence of hydrogen at from 3 to 8 MPa and a temperature of from 40 to 150° C., preferably from 90 to 130° C., and distillatively working up the reaction mixture to remove NH3, H2O and by-products. When an Ru supported catalyst is used, high cis/trans isomer ratios of 84/16 (total yield of IPDA: 81%) are achieved.
DE-A 43 43 891 describes a process for preparing IPDA by reacting IPN with hydrogen in the presence of ammonia and a suspension or fixed bed hydrogenation catalyst from the group of cobalt, nickel and noble metal catalysts at a pressure of from 3 to 20 MPa and a temperature of up to 150° C., and distillatively working up the reaction mixture. The reaction is carried out in two stages, and precisely defined temperature ranges have to be observed for the individual stages. A cis/trans isomer ratio of 80/20 can be achieved at an overall IPDA yield of 91.9%.
In the process of EP-A 0 926 130, the hydrogenation is carried out in the presence of an acid over catalysts which comprise copper and/or a metal of the eighth transition group of the periodic table. Both Lewis and Brönstedt acids are used; preference is given to using 2-ethylhexanoic acid. The addition of acid increases the cis/trans isomer ratio. The cis/trans isomer ratios are generally ≧70/30 at an overall IPDA yield of ≧90%.
The process of EP-B 0 729 937 is notable in that the process is carried out in three spatially separated reaction chambers using cobalt, nickel, ruthenium and/or other noble metal catalysts. Upstream of the second reactor, aqueous NaOH solution is metered in, which reduces the formation of cyclic by-products such as 1.3.3-trimethyl-6-azabicyclo[3.2.1]octane.
In the process of DE-A 101 42 635.6, which has an earlier priority date but was unpublished at the priority date of the present invention, IPDA having a cis/trans isomer ratio of at least 70/30 is obtained, starting from IPN, by using a hydrogenation catalyst in the hydrogenation step which has an alkali metal content of ≦0.03% by weight, calculated as the alkali metal oxide.
A disadvantage of the existing processes for preparing IPDA having a high cis content is the costly and inconvenient preparation of the catalysts used. In addition, these catalysts generally suffer from aging, which reduces their catalytic activity in the course of time. In order to compensate for this, the reaction temperature is usually increased, which leads, however, to a deterioration in the cis/trans isomer ratio and the selectivity and therefore to an increase in the formation of by-products. In addition, most of the processes known from the prior art are notable for a complicated reaction procedure.
A process for preparing isophoronediamine having a high trans/cis isomer ratio can be taken from DE-A 42 11 454. In this process, trans-isophoronediamine is prepared from isophoronenitrile via isophoronenitrile azine. It is also described that trans-isophoronediamine would be obtained by distilling the commercially obtainable cis/trans isomer mixture. However, since the cis-isomer occurs as the main product, this process is uneconomic.